The development of alkaline phosphatase activity in the mouse duodenum is characterized by the progressive appearance of a number of different molecular forms of the enzyme. These isozymes are localized in the brush border of the epithelial cells lining the villi, and their sequential appearance occurs during a morphological differentiation of the microvilli. It is the objective of the proposed investigation to purify these isozymes by the use of affinity chromatography, and to compare them with respect to their composition and structure. The information obtained from this study should provide a deeper insight into the nature of the molecular changes that occur in the enzyme during the differentiation of the epithelial cells.